The physical fitness and strength requirements for lower limb amputee gait are so high that thirty-seven percent of amputees are unable to walk with a prosthesis and another twenty-three percent are able to ambulate indoors only. Recent advances in prosthetic feet and knees have been unable to reduce these requirements, and external assistive devices like powered exoskeletons are too expensive and too power-intensive to be considered feasible solutions for use with this population. Passive assistive devices may be a feasible alternative to powered exoskeletons to improve mobility for lower limb amputees. The goal of this project is to create such a device by utilizing exotendon technology to create a passive prosthesis to reduce metabolic cost and muscular fatigue for lower limb amputees. Exotendons are passive elastic structures mounted in parallel to a limb that stretch across pulleys placed at the joint centers. Borrowing from horse anatomy where short muscles interact with long tendons that stretch across multiple joints, exotendons can store energy and return it at a different time or even to a different joint when positive power is needed. Additionally, they are able to contribute to muscle force production without adding to the metabolic cost. In this Phase I work, we propose to develop an exotendon prosthesis for transfemoral amputees through three Specific Aims. In Specific Aim 1, we will determine optimum design parameters through biomechanical modeling. In Specific Aim 2, we will design and fabricate an exotendon prosthesis. And in Specific Aim 3, we will conduct a pilot study with transfemoral amputee subjects to investigate the performance of the exotendon prosthesis. Future work in Phase II will focus on refining and expanding the exotendon prosthesis through the following aims. In Specific Aim 1 of Phase II, we will model the use of exotendons with transtibial amputees and develop a transtibial exotendon prosthesis. In Specific Aim 2 of Phase II, we will further develop the exotendon prosthesis design to provide even greater kinetic and metabolic benefits through the use of non-uniform radii pulleys for both transfemoral and transtibial amputees. In Specific Aim 3 of Phase II, we will evaluate the performance of the exotendon prostheses in a trial with a large enough sample size to permit statistical comparisons. Specifically, we will perform laboratory experiments to determine the reductions in joint moments and metabolic cost with the use of exotendons and we will also perform field experiments to investigate the ability of the exotendon prosthesis to improve activity level. Lastly, in Specific Aim 4 of Phase II, we will perform a case series to evaluate the ability of the exotendon prosthesis to facilitate walking for amputees with limited abilities to ambulate.